Manufacture of armor plates



Dec. 21, 1937. WLLER 2,102,963

MANUFACTURE OF ARMOR PLATES Filed May 28, 1936 2 Sheets-Sheet 2 l NVENTOR $50.99? [5f M451? ATTORNEY Patented Dec. 2l, 1937 UNITED STATES PATENT OFFICE 15 Claims.

The present/invention relates particularly to the manufacture of plates or sheets of metal intended for armor plate such as used in the con- ,struction of tanks or armored planes, battle- 5 ships or other war machines. While the invention has been developed in response to the need for manufacture of armor plate, it is apparent that it may have a wider and broader adaptability and utility. l

The present invention has been brought abou -by the demand for an integral bimetallic armor plate for use in the manufacture of tanks and the practical difficulties in producing sheets or plates arising frm the nature of the material.

For use as armor plate asheet steel has been developed in which the sheets or plates are composed` of a bimetallic structure, the outer shell or case being composed of a steel with a high carbon content so as to resist penetration of 20 projectiles, and the inner layer or'core is composed of a softer backingl of a steel with a much lower carbon content. The outer case or shellis made by carburizing at high temperature While the inner face or core is protected or 5 shielded from the effects of the carburizing treatment. The resultant sheet is then composed of an outer layer of steel having varying carbon contents and known as austenite, .martensite, troostite and sorbite, nally merging intothe soft steel 4center or core. A steel plate thus treated resists the penetration .of the projectile, but if penetrated the metal of the sheet will either be cut out as a clean, solid slug or will bend up and deform without breaking off and scattering. The result is that no fragments or particles of metal are scattered through the interior of the armored compartment or chamber.

The invention set forth herein has as one of its objects a new and improved method of treatlng steel plates in the carburizing process by which a sheet or plate is effectively carburized on the outer face, but the inner face is protected. Theprocess of carburizing is one which is simple and eective.

After the sheets are carburized, they are rst annealed and then subjected to a heat treatment by which the surface is fully hardened and the plates finished. The diiiiculty with the manufacture of these plates has been that in the processes of annealing and tempering, the bimetallic character of the metal sheets has caused them to warp or bendand buckle to such an extent that they are either unfit for subsequent working or, if treated in the usual fashion, they are either destloyed orinjured to such an extent as to be worthless for their intended purposes.

It has been impracticable before my invention to make armor plate of a composite structure, 6 such as described, economically1 and efficiently. In fact, the distortions and stresses set up in the plates have greatly interfered with the use of such metal sheets and have made it practically impossible to work sheets of this character in 10 the manufacture of tanks, airplane fuselages and the like. The present invention, however, presents a new and radically dierent process for the treatment of this material by which itis possible to secure flat plates without stresses or distortion and which can be practically and successfully* used in the fabrication of all sorts of armored structures. The method of treatment described herein is one which has been successfully .used in the manufacture of bimetallic armor plate.

It will be understood and appreciated that in the detailed description of t'ne invention contained herein, the process has been described in its best known embodiment and in such detail as to permit those skilled inthe art to understand and practice the invention, but the process is not limited to exact compliance with the de-A tails and may be applied in other specic forms.

It will also be understood that modifications and 3G improvements may be made without departing from the essential features of the invention.

In the drawings accompanying the applica.- tion, several steps of the process are shown in more or less diagrammatic form, the specication being a full de scriptionof the' entire process.

In these drawings:

Figure 1 is a view, partly in section, of two steel plates as they are assembled for the carburizing treatment by which the outer surface is transformed into steels of high carbon content, having maximum resistance to penetration, While the interior or rear face and core of the steel sheet remains in substantially its original 4,. composition; j

Figure 2 is a cross-section of a sheet which has been treated, showing the outer case and the interior core;

Figure 3 shows the first step in the process of 50 straightening a sheet of thiscomposite bimetallic structure which is performed after the quenching operation and by which the stresses and strains are distributed throughout the metal sheet as it is cooling;

Figure 4 shows in cross-section the form which the sheet takes after it leaves the operation shown in Figure 3 and while it is still hot;

Figure 5 shows the form which the sheet as- The complete process of manufacture of a composite bimetallic steel plate is described herein, reference being made first to the method employed for carburizing the plates and thus creating the outer case of very hard, penetrationresistant steel. vFor this purpose it is proposed to proceed in an' original, novel and effective fashion.

In Figure 1 the numerals I and 2 represent two plates of steel which are to be treated and transformed into bimetallic armor plate, as they are assembled for the carburizing process. The plates are usually formed with many holes through which rivets or other securing means are passed or which are drilled for receiving brackets and other attachments. These holes are utilized for the reception of clamping bolts 3 by which the plates are held together with the faces exposed which are to be carburized to give the outer shell. The inner surfaces are thus protectedfrom the carburizing elements and remainv in their original relatively soft condition. Usually the plates are made in rights and lefts so that two plates can be accurately matched, but in case this is not practical a dummy canlbe clamped to one side of-a single plate to protect its inner surface. Tapered washers 4 are used at the bolts which expose the surface of the plate all around the holes so that the carburizing process will extend over the entire outer surface of the plate to be treated.

Between the plate is placed a layer of plastic material which serves as a carbon resistant and seals the opposed surfaces ofthe plates. For this purpose may be employed a slurry or paint composed of a good grade of re clay, asbestos-and sodium silicate. The surfaces of the plates which are to be protected may be given one or more coats of this compound and when the last coat is partially dried, the plates are secured together as firmly as may be possible. This intermediate protective layer is indicated by thel numeral 5. Other compositions of carbon resistant may be employed.

When the plates have been prepared and assembled as indicated, they are packed in boxes for the cementation or carburizing process. For this purpose they are packed in a high carbonaceous material which will liberate carbon gases under high temperatures. The plates are carefully packed so that no plates are in contact and the boxes tightly sealed so that the flames from the furnace cannot come into direct contact with the carburizing material. Boxes are then introduced into the furnace and the temperature raised slowly until it reaches the proper point, usually about 1660 F. The time and temperature are largely determined by they skill of the operator, as such process is known in the art and need not Vbe further detailed. The effect of the carbon is to alter and modify the steel structure at the exposed surfaces and into the body of the steel for a sufficient distance to secure the outer hardened case which gives the penetration resistance to the armor plate.

After the carburizingprocess has been carried on for the time required, dependent upon the results desired, the furnace is allowed to cool until the boxes can be handled. The boxes are then opened and the plates allowed to cool to atmosphere, whereupon the bolts 3 are taken out and the plate is ready for subsequent treatments.

One of theplates as it appears after the end of the carburizing process is shown in Figure 2. It is now composed of the inner core 0r backing 1 of relatively soft steel and the outer casing of high carbon steel shown at 8. It will be observed that the edges of the plate where they have been exposed to the treatment are also converted into the hardened metal. rThe protective coating 5 has baked upon the plate and is now removed.

The next step in the process is the annealing of the plates. This is done in an annealing furnace where the temperature is raised to between 1100 and 1200 F. The plates are held at this temperature for approximately an h'our and then allowed to cool without exposure to the air until they are down to approximately atmospheric temperature.

It is desirable to remove the hardened case -around the edges of the plates after the annealing operation. This is done by cutting the edges along the lines 9, care being taken to remove all of the hardenedcase. This is very essential to the success of the subsequent steps of the process for the outer case and the inner core have very different coefficients of expansion, and unless these edge portions were removed they would bind the sheet and cause extreme distortion and buckling which it would be impossible to remove.

The plates are now ready for the na1 heat treatment. This is d'one by heatingthe plates at a temperature slightly above the critical temperature of the steel for a sufficient period to allow the plates to be thoroughly heated throughout the entire mass. The exact time required will vary with the size and thickness of the plate. The plates are then removed and immediately quenched in a bath of quenching oil to atmospheric temperature. During this process the oll should be thoroughly circulated and agitated to prevent the formation of soft spots due to steam or gas pockets. The plates are then treated to a second heating at approximately the same temperature and time and are then given a second quenching treatment.

when the plate is removed it has not reached atmospheric temperature, but considerable heat is retained within the plate. The time is again dependent upon the size and thickness of the plate and may be determined by empirical tests familiar to those skilled in the art. Usually a second quenching of a few seconds will give desirable results.

When the plate is now removed there is very considerable heat retained within the plate, and it is ready for the rst of a series of treatments by which it-is assured that the plate upon cooling will be free of irregularities, buckled or warped places. Immediately upon removing the hot plate, it is introduced into a roller leveler such, for example, as that shown in Figure 3 and is passed'back and forth through this roller leveler for a period of from five to fifteen minutes, depending upon the size and thickness of In this second quenching operation, however, the time is reduced materially so that the plate. Pressure, depending upon the size and thickness of the plate, is exerted upon the plate during the rolling operation by the series of rollers l2 and the plate is given as many passes between the rollers as may be practicable during the time allowed.

The purpose of the rolling operation is to distribute the internal strains and stresses throughout the bimetallic plate as it cools down toward atmospheric pressure. It will be found that when the plate is taken from the quenching bath it develops considerable distortion due to the bimetallic structure of the plate, and the object of the rolling and leveling operation is to prevent localization of these stresses during cooling and to insure an even distribution of the various forces set up by the differences in coeli'cients of expansion between the outer case and the inner core. The rolling and leveling operation is stopped before the plate has been completely cooled, and it will be found that when it is laid on the floor it assumes a generally concave shape with the outer case 8 on the concave side of the plate. If the rolling operation has been continued for the proper length of time so that the stresses and strains are evenly distributed, the plate will have an approximately even and regular concavity. The condition of the plate at this time is shown in Figure 4.

As the plate cools down, it reverses itself or turns itself inside out, the hard side 8 being on the convex side and the soft side 'I on the concave side. This is shown in Figure 5. The degree of concavity in the cooled plate is variable and the explanation for the peculiar behavior of the plate is not clear. The success of the process as set forth is believed to lie in the fact that in the heat treating and rolling operations excessive stresses are put into the plate which causes it, when cooled, to assume the position of Figure 5, in which the hard side of the plate is under tension and the soft side under compression. This is extremely important in the process of restoring final equilibrium to the sheet so that it will be flat in its final form, for in the nal 'strengthening step, the hard side of the sheet is placed in compression and the soft side under tension. Attempts to straighten a plate in which the reverse conditions exist result in cracking the plate.

The final straightening operation is preferably carried out in whatis generally known as a break press, such as illustrated Ain Figures 6 and '7. This press comprises a frame l5 in the upper part of which is mounted the rotating cam shaft I6 which reciprocates the head or ram I1. On the lower part of the ram is secured the male die or hammer I8 and on the frame is supported the female die. The stroke of the male die is regulated at I9. The female die is a long troughshaped plate which extends across the machine and is'somewhat larger than the greatest width of the sheet to be treated. The male die is relatively narrow so that it will operate upon a limited area of the sheet. The female die is` convex and the male die concave. In operation, the cooled sheet is introduced between the dies with the convex or hard side uppermost and fed across the die so that the entire area of the sheet is acted upon. The male die is adjusted so that at its lowermost point it enters slightly into the female die and will bend the sheet reversely to approximately the extent indicated by the dotted line 20.

By the succession of blows the hard surface of the sheet is placed in compression while the soft side is stretched sumciently to create the equilibrium of forces and stresses in the sheet and it will be flattened out so that it is practicable to work the finished sheet.

' As indicated in Figure '7, the male die I8 may be shifted across the machine so that every part of the plate is subjected to the blows of the die.

It will be understood that heretofore it has been impossible to produce commercially armer plate of the composite type described herein,and the present process was originated and perfected for the purpose of permitting the successful manufacture of these sheets to meet the requirements for the manufacture of tanks. The extreme distortions occurring in these sheets have made the manufacture of this type of metal a practical impossibility which has been solved by the inventor.

If the plates are of angular or curved sections, the heat treating operation is similar except that instead of using the roller leveler the plate is bolted to a rigid form and quenched in this form. This operation will secure the distribution of stresses which is so essential to the process. also possible to air-quench the plates, but this will not produce as satisfactory results for the hard surface secured by air-quenching is not of as high quality as an oil-quenched plate. Y

As an example of the results secured by the process herein described, the hard side of the plate will show a Brinell hardness from 514 to 600, depending upon the thickness of the plate, while the soft side will show a hardness from 235 to 444, depending upon the same factors.

What is claimed is:

1. In the manufacture of armor plate, the steps of carburizing one surface of a steel plate to form a penetration-resistant case and relatively soft core, annealing the plate, reheating the plate, partially quenching the plate and while'the plate retains a substantial part of the heat, distributing the stresses uniformly throughout the plate so that when the same is cooled it assumes a substantially uniform concavo-convex surface with the case on the convex side thereof, and straightening the plate by compressing the case and stretching the core.

2. In the manufacture of armor plate, the steps of carburizing one surface of a steel plate to form d a penetration-resistant case and-a relatively soft core, annealing theplate, reheating the plate, partially quenching the plate and while the plate retains a substantial part of the heat, distributing the stresses uniformly throughout the plate by repeated rolling operations so that when the plate is cooled it assumes a substantially regular and uniform curvature with the case on the convex side thereof, and straightening the plate by compressing the case and stretching the core.

3. A process of straightening a bimetallic armor plate having a hardened case and a relatively soft core, comprising heating the plate and partially quenching it, and while the plate is still hot, rolling it to distribute the stresses uniformly, and then straightening the plate by placing the case under compression andthe core under tension.

4. A process of straightening a bimetallic arthe plate to cool whereby it will assume a regular curvature with the case on the convex side of the plate, and then flattening the plate.

5. A process of straightening a bimetallic armor plate havinga hardened case and a relatively soft core, comprising heating the plate and then quenching the same, removing the plate from the quenching medium before it loses a substantial portion of its heat, repeatedly rolling the plate while cooling and then permitting the plate to cool whereby it will assume a regular curvature with the case on the convex side of the plate, and then flattening the plate by repeated blows on the convex side thereof.

6. A process of straightening a bimetallic armor plate having a hardened case and a relatively soft core, comprising heating the plate and then partially quenching the same, working the plate while still hot to distribute the stresses uniformly throughout the plate, permitting it to cool so that it assumes a regular concavo-convex form with the case on the convex side thereof,

vand flattening the plate.

7. A process of straightening a bimetallic armor plate having a hardened case and a relatively soft core, comprising heating the plate and then partially quenching the same, working the plate while still hot to distribute the stresses uniformly throughout the plate, permitting it to cool so that it assumes a regular concavo-convex form with the case on the convex side thereof, and fiattening the plate by repeated blows on the case side.

8. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and quenching the plate, and distributing the stresses setup by the cooling of the plate throughout the entire area thereof.

9. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at-high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, and distributing the stresses set up during the cooling of the plate throughout the entire area thereof.

10. In the process of manufacturinga-bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the pla-te with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, distributing the stresses set up during the cooling of the plate throughout the entire area thereof, and fiattening the plateby compressing the case and tensioning the core.

11. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, dis'- tributing the stresses set up during the cooling of the plate throughout the entire area thereof, and subjecting the plate to a multiplicity of impacts against the convex or case side of the plate.

12. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and quenching the plate, and distributing the stresses set up by the cooling of the plate throughout the entire area thereof by repeated rolling operations,

13. In the process of manufacturing a bimetallicv armor plate having a high carbon case and a. relatively low carbon core,the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, and distributing the stresses set up during the cooling of the plate throughout the entire area thereof by repeated rolling operations.

14. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, distributing the stresses set up during the cooling of the plate throughout the entire area thereof by repeated rolling operations, and flattening the plate by compressing the case and tensioning the core.

l5. In the process of manufacturing a bimetallic armor plate having a high carbon case and a relatively low carbon core, the steps of coating the core side of the plate with a carbon-resistant plastic, covering the core side with a protective sheet, subjecting the plate to a carburizing agent at high temperature to cause the formation of the outer case on the exposed surface of the plate, removing the case from the edges of the plate, heating and partially quenching the plate, distributing the stresses set up during the cooling of the plate throughout the entire area thereof by repeated rolling operations, and subjecting the plate to a multiplicity of impacts against the convex or case side of the plate.

GEORGE LEE MILLER. 

